The invention relates to an electric motor-supported steering system.
The term electric motor-supported steering devices includes active steering systems in which an electric motor has a driving action on a differential drive in order to superimpose a steering movement on the steering movement initiated by the driver depending on the driving situation. Such a steering device is known for instance from DE 39 08 217 A1. Furthermore, the aforesaid preamble also includes steering devices in which the task for the electric motor is merely to support the steering movement initiated by the driver by making available auxiliary steering force. In such steering systems, the electric motor is frequently drivingly connected to a rack of a steering gear.
The electric motors, embodied for instance as synchronous or asynchronous motors, are controlled using power electronics with a frequency converter. It converts the supply voltage from the vehicle onboard electric system to a plurality of clocked pulse-width modulated signals with frequencies in the 20 kHz range. The mean of the pulse-modulated signals is set such that the motor current desired depending on application is set via the inductivity of the electric motor. Depending on application, the motor current can be for instance sinusoidal or rectangular. The power electronics contains a plurality of power transistors. The switching processes performed thereby necessarily result in the transmission of interfering signals that affect electromagnetic compatibility (EMC). These are conducted for instance via the lines to the electric motor, the battery line for the steering device, and other electric connections on the steering device and there cause interfering emissions. In addition to the direct interfering emissions, there is overcoupling onto other lines in the motor vehicle and this causes further interfering emissions and/or galvanic interfering transmission to the onboard electric system.
Minor EMC interference results from long switching times, that is, from long control voltage rise and/or fall times and/or from long control current rise and/or fall times for building up and reducing the capacities of the power electronics or power transistors, but the power loss in the power electronics during long switching times is great, which can lead to the power electronics heating up sharply and thus in extreme cases to a functional failure of the power electronics and thus of the electric motor. When dimensioning the control of the electric motor, the switching times are set or established as slow as is permissible for a theoretical maximum ambient temperature or maximum power electronics temperature. That is, the design is based on the maximum permissible power loss and thus the maximum permissible heating at a theoretical maximum temperature.
The underlying object of the invention is to improve the EMC in a steering device.